A rocket engine for single-stage-to-orbit (SSTO) application must operate at high thrust at lift off, moderate thrust later in the flight, and very low thrust (i.e., .apprxeq.10%) during vehicle landing. Reduced thrust operation involves reduced flow rates of the propellants and therefore reduced pressure in the combustion chamber.
Conventional liquid propellant rocket engines use the fuel propellant (e.g., hydrogen) to regeneratively cool the walls of the combustion chamber and nozzle(s). The heat transfer involved in this process and the work done in pumping the fuel to the combustion chamber, nozzle(s), and propellant injector assembly adds sufficient heat to the fuel such that it is converted from a liquid to a high pressure compressible fluid. Thus, as the pressure in the combustion chamber is reduced (i.e., as flow rates are reduced) from high pressure at full thrust to low pressure at low thrust, the density of the compressible fuel is correspondingly reduced and the injection velocity of the fuel exiting the fuel injector(s) of the propellant injector assembly into the combustion chamber remains approximately constant.
However, the oxidizer (e.g., oxygen) is not heated enough by heat transfer and/or by being pumped to the propellant injector assembly to cause it to be converted from a liquid to a compressible fluid. Thus, as pressure in the combustion chamber is reduced from high pressure at full thrust to low pressure at low thrust, the density of the oxidizer injected into the combustion chamber by the conventional oxidizer injector(s) of the propellant injector assembly remains approximately constant. This results in the pressure drop of the oxidizer across these injector(s) being proportional to the square of the flow rate. For example, with a 100 psi pressure drop at 100% flow rate (i.e., full thrust), the pressure drop at 10% flow rate (i.e., very low thrust) is 1 psi (100.times.(0.10).sup.2). Such a pressure drop at very low flow rate is insufficient to provide uniform flow distribution and stable combustion.